The present invention relates to motor drives for variable speed control of AC induction motors and more particularly to a method and apparatus for smoothly starting an AC motor drive into a rotating motor.
Induction Motors Induction motors have broad application in industry, particularly when large horsepower is needed. A typical induction motor includes a stator and a rotor. The stator includes a three phase stator winding which forms a cylindrical stator cavity. A common rotor design includes a "squirrel cage winding" in which axial conductive bars are connected at either end by shorting rings to form a generally cylindrical structure. The rotor is concentrically mounted for rotation within the stator cavity.
The rotor is forced to rotate within the stator cavity by providing three phase electrical voltages to the stator windings. The stator voltages generate stator currents which in turn cause a rotating magnetic stator field. The stator field interacts with the rotor to cause rotation.
As with movement of any object, rotor movement requires both (1) interaction between the stator field and the rotor and (2) a force applied to the rotor. Without enough interaction, even an extremely large force cannot move a rotor. Similarly, without enough force, even mechanical contact could not move a rotor.
The interaction required for rotor movement is provided as follows. As the stator field rotates about the cavity, stator field flux lines cut across rotor bars. If the stator field rotates at a speed which is slightly greater than a rotor speed so that each bar is subjected to a slowly varying stator field, the stator flux induces rotor bar currents (hence the term "induction" motor). The term "slip" will be used hereinafter to refer to the difference between the stator field and rotor frequencies.
The rotor currents cause a magnetic rotor flux field. Rotor field strength is related to the number of stator flux lines cut by the rotor bars and therefore the amounts of rotor and stator flux are both related to stator field strength. The stator and rotor fields attract and hence provide the interaction required for rotor movement. Thus, the stator field has an "attraction" component which provides the interaction required for rotor movement. Because the stator field attraction component causes flux which interacts with the rotor, the attraction component is referred to hereinafter as the "flux producing" component.
The force required for rotor movement is provided as follows. As the stator field rotates about the cavity, if stator and rotor field attraction is sufficient, the stator field "pulls" the rotor along thereby causing rotation. Thus, in addition to the flux producing component, the stator field has a "shear force" component pushing tangent to the rotor surface tending to rotate the rotor. Because the shear force causes a rotating torque on the rotor, the shear force is referred to hereinafter as the "torque producing" component.